U.S. patent number 7,599,740 [Application Number 11/748,659] was granted by the patent office on 2009-10-06 for ventricular event filtering for an implantable medical device.
This patent grant is currently assigned to Medtronic, Inc.. Invention is credited to Paul A. Belk, Robert A. Betzold, David A. Casavant, Steven R. Hornberger, Thomas J. Mullen, Douglas A. Peterson, Todd J. Sheldon, John C. Stroebel.
United States Patent |
7,599,740 |
Betzold , et al. |
October 6, 2009 |
Ventricular event filtering for an implantable medical device
Abstract
Pacing parameters are provided to address cross talk and
intrinsic ventricular events occurring within a predefined blanking
period following an atrial event. The parameters are used in
conjunction with protocol for minimizing or reducing ventricular
pacing, wherein ignoring intrinsic ventricular events during the
blanking period might otherwise affect the performance of the
protocol.
Inventors: |
Betzold; Robert A. (Fridley,
MN), Casavant; David A. (Reading, MA), Belk; Paul A.
(Maple Grove, MN), Mullen; Thomas J. (Ham Lake, MN),
Stroebel; John C. (Blaine, MN), Hornberger; Steven R.
(Minneapolis, MN), Sheldon; Todd J. (North Oaks, MN),
Peterson; Douglas A. (Apple Valley, MN) |
Assignee: |
Medtronic, Inc. (Minneapolis,
MN)
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Family
ID: |
35064713 |
Appl.
No.: |
11/748,659 |
Filed: |
May 15, 2007 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20070213777 A1 |
Sep 13, 2007 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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10850666 |
May 21, 2004 |
7245966 |
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10246816 |
Sep 17, 2002 |
7130683 |
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09746571 |
Dec 21, 2000 |
6772005 |
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Current U.S.
Class: |
607/9 |
Current CPC
Class: |
A61N
1/368 (20130101); A61N 1/3702 (20130101); A61N
1/3688 (20130101) |
Current International
Class: |
A61N
1/362 (20060101) |
Field of
Search: |
;607/9,14,27
;128/901 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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0363015 |
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Apr 1990 |
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0448193 |
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Sep 1991 |
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EP |
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0624386 |
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Nov 1994 |
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EP |
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0830877 |
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Mar 1998 |
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EP |
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1449562 |
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Aug 2004 |
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EP |
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WO 95/32758 |
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Dec 1995 |
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WO |
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WO 02/051499 |
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Jul 2002 |
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WO |
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WO 2005/097259 |
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Oct 2005 |
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WO |
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WO 2005/113065 |
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Dec 2005 |
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WO |
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WO 2006/079037 |
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Jul 2006 |
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WO |
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WO 2006/079066 |
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Jul 2006 |
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WO |
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Primary Examiner: Layno; Carl H.
Assistant Examiner: Morales; Jon-Eric C.
Attorney, Agent or Firm: Bauer; Stephen W.
Parent Case Text
CROSS REFERENCE TO RELATED APPLICATIONS
The present application is a divisional application of Ser. No.
10/850,666, filed May 21, 2004, now U.S. Pat. No. 7,245,966, which
is a continuation-in-part of Ser. No. 10/246,816, filed Sep. 17,
2002, now U.S. Pat. No. 7,130,683 which is a continuation-in-part
of non-provisional U.S. patent application Ser. No. 09/746,571
filed Dec. 21, 2000, now U.S. Pat. No. 6,772,005 both of which are
herein incorporated by reference in their entireties.
Claims
The invention claimed is:
1. A method of filtering far field sensing from intrinsic events
occurring during an atrial blanking period that follows an atrial
event, the method comprising: monitoring a ventricular channel
during the atrial blanking period for a first interval; classifying
a first sensed event on the ventricular channel during the atrial
blanking period of the first interval as invalid; monitoring the
ventricular channel during the atrial blanking period of a second
interval, subsequent to the first interval; and reclassifying the
first sensed event as valid if the atrial blanking period of the
second interval transpires without a sense on the ventricular
channel.
2. A method of filtering far field sensing from intrinsic events
occurring during an atrial blanking period that follows an atrial
event, the method comprising: monitoring a ventricular channel
during the atrial blanking period for a first interval; classifying
a first sensed event on the ventricular channel during the atrial
blanking period of the first interval as valid; monitoring the
ventricular channel during the atrial blanking period of a second
interval, subsequent to the first interval; and classifying a
second sensed event on the ventricular channel during the atrial
blanking period of the second interval as invalid in response to
the occurrence and classification of the first sensed event.
Description
FIELD OF THE INVENTION
The present invention generally relates to implantable medical
devices and more specifically to implantable medical devices for
providing cardiac pacing.
BACKGROUND OF THE INVENTION
In a wide variety of commonly employed dual chamber pacing
modalities, cross-talk could cause one or more errors. For example,
a paced atrial event may be sensed by a ventricular lead and
misinterpreted as a ventricular event. This would effectively be
far field sensing of an atrial pace. This typically would not be a
problem with intrinsic atrial depolarizations, due to their lower
magnitude. Conversely, far field sensing of intrinsic R waves or
paced ventricular events could likewise be misinterpreted if sensed
by an atrial lead.
To account for such errors, various blanking or refractory periods
are employed such that these events are either not sensed or are
simply ignored. In dealing with far field sensing of atrial-paced
events, ventricular events that are sensed during a given window
following the atrial pace are ignored. Depending upon the
application, this window may be referred to as the atrial blanking
period (ABP) or some similar nomenclature. Through clinical
application, practitioners have determined that if such far field
sensing is going to occur, it typically happens within 80 ms or
less of the original event (e.g., the atrial pace). Thus, this
window is conservatively set at 80 ms or so, depending upon the
specific device or the manufacturer.
In use, such as in a DDD mode, providing this window adequately
addresses the cross-talk problem and generally does not create
additional problems. Sometimes genuine intrinsic events will occur
during the window and will also be ignored. For example, a
premature ventricular contraction (PVC) is an intrinsic, conducted
event but if it falls within the window it will be ignored.
Whether a far field sense or an intrinsic event, such as a PVC,
occurs and is ignored, the subsequent action of the device in
typical dual chamber modes is to provide a ventricular pace at the
expiration of a predetermined interval following the initial atrial
event, unless inhibited. If the ignored event was cross talk, it is
certainly possible that a subsequent intrinsic ventricular event
will occur and inhibit the pace. Alternatively, for any number of
reasons no intrinsic event will occur during the atrial-ventricular
interval (AVI) and the ventricular pace is delivered. If the
ignored event was a PVC, it is quite likely that there will not be
another intrinsic ventricular event in the current A-A interval and
the device will deliver a pacing pulse at the expiration of the
AVI.
Thus, the use of such a window in dual chamber devices is
appropriate to prevent cross talk without introducing additional
problematic results. As disclosed in the above referenced
applications, a mode and/or protocol is provided that minimizes or
greatly reduces ventricular pacing and is referred to as MVP. In
summary, MVP tolerates a complete cycle (A-A) interval without
ventricular activity, in order to promote intrinsic conduction. In
many patients, the conduction pathway is intact but is delayed
beyond the capabilities of traditional dual chamber mode timing.
Thus, ventricular pacing is provided when not absolutely necessary
and this is believed to be undesirable.
Various embodiments of MVP are described in greater detail in the
referenced applications, but the mode generally operates by
monitoring a complete cycle for intrinsic conduction. If intrinsic
conduction fails and no ventricular event occurs, ventricular
pacing is provided in the subsequent cycle.
The use of the above described window (ABP) presents a challenge to
this minimized or reduced ventricular pacing mode. For example, if
true cross talk occurs and is ignored, subsequent operation
continues unhindered. However, if a PVC occurs during this window,
it is ignored. Thus, a true intrinsic ventricular event is being
ignored by a mode that bases it subsequent operation on the
presence or absence of intrinsic ventricular activity during a
given cycle. If a PVC occurs during this window it is ignored;
assuming no other ventricular activity occurs during this interval,
which is quite possible, the device determines that the current A-A
interval is devoid of intrinsic ventricular conduction.
Subsequently, the device mode switches or otherwise operates to
deliver ventricular pacing in a subsequent cardiac cycle and
depending upon the embodiment, one or more subsequent cycles. While
not in and of itself harmful, this ventricular pacing is generally
not necessary as intact conduction exists. As a result, PVC's may
operate to reduce the efficiency of the ventricular minimization or
reduction protocol insomuch as that efficiency is determined to be
the reduction or elimination of otherwise unnecessary ventricular
pacing. This may simply result in a relatively low number of
unnecessary ventricular paces. Alternatively, depending upon the
specific embodiment of MVP, a series of PVCs may be interpreted as
a loss of conduction that prevents a return to the atrial based
mode for a longer period of time.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an illustration of a body-implantable device system in
accordance with the present invention, including a hermetically
sealed device implanted in a patient and an external programming
unit.
FIG. 2 is a perspective view of the external programming unit of
FIG. 1.
FIG. 3 is a block diagram of the implanted device from FIG. 1.
FIG. 4 is a ladder diagram of the ADI/R operation.
FIG. 5 is a ladder diagram of the committed DDD/R operation in the
event that the patient develops transient AV block.
FIG. 6 is a ladder diagram that depicts the pacing operation in the
event that the patient develops AV block that persists for more
than one cycle.
FIG. 7 is a ladder diagram that depicts a periodic attempt to
restore the ADI/R operation during sustained DDD/R operation.
FIG. 8 is a ladder diagram of the pacing operation in the event
that the patient develops an atrial tachycardia.
FIG. 9 is a flow chart illustrating one embodiment of a mode
supervisor according to the present invention.
FIGS. 10A-10B are ladder diagrams that illustrate ventricular sense
events during an ABP.
FIGS. 11A-11B are ladder diagrams that illustrate ventricular sense
events during an ABP, while utilizing a feed back protocol.
FIGS. 12A-12B are ladder diagrams that illustrate ventricular sense
events during an ABP, while utilizing a feed forward protocol.
DETAILED DESCRIPTION OF THE DRAWINGS
FIG. 1 is an illustration of an implantable medical device system
adapted for use in accordance with the present invention. The
medical device system shown in FIG. 1 includes an implantable
device 10--a pacemaker in this embodiment--that has been implanted
in a patient 12. In accordance with conventional practice in the
art, pacemaker 10 is housed within a hermetically sealed,
biologically inert outer casing, which may itself be conductive so
as to serve as an indifferent electrode in the pacemaker's
pacing/sensing circuit. One or more pacemaker leads, collectively
identified with reference numeral 14 in FIG. 1 are electrically
coupled to pacemaker 10 in a conventional manner and extend into
the patient's heart 16 via a vein 18. Disposed generally near the
distal end of leads 14 are one or more exposed conductive
electrodes for receiving electrical cardiac signals and/or for
delivering electrical pacing stimuli to heart 16. As will be
appreciated by those of ordinary skill in the art, leads 14 may be
implanted with its distal end situated in the atrium and/or
ventricle of heart 16.
Although the present invention will be described herein in one
embodiment which includes a pacemaker, those of ordinary skill in
the art having the benefit of the present disclosure will
appreciate that the present invention may be advantageously
practiced in connection with numerous other types of implantable
medical device systems, and indeed in any application in which it
is desirable to provide the preferred ADI/R pacing mode (i.e., the
MVP modality), as may occur in ICDs and the like.
Also depicted in FIG. 1 is an external programming unit 20 for
non-invasive communication with implanted device 10 via uplink and
downlink communication channels, to be hereinafter described in
further detail. Associated with programming unit 20 is a
programming head 22, in accordance with conventional medical device
programming systems, for facilitating two-way communication between
implanted device 10 and programmer 20. In many known implantable
device systems, a programming head such as that depicted in FIG. 1
is positioned on the patient's body over the implant site of the
device (usually within 2- to 3-inches of skin contact), such that
one or more antennae within the head can send RF signals to, and
receive RE signals from, an antenna disposed within the hermetic
enclosure of the implanted device or disposed within the connector
block of the device, in accordance with common practice in the
art.
FIG. 2 is a perspective view of programming unit 20 in accordance
with the presently disclosed invention. Internally, programmer 20
includes a processing unit (not shown in the Figure) that in
accordance with the presently disclosed invention is a personal
computer type motherboard, e.g., a computer motherboard including
an Intel Pentium 3 microprocessor and related circuitry such as
digital memory. The details of design and operation of the
programmer's computer system will not be set forth in detail in the
present disclosure, as it is believed that such details are well
known to those of ordinary skill in the art.
Referring to FIG. 2, programmer 20 comprises an outer housing 60,
which is preferably made of thermal plastic or another suitably
rugged yet relatively lightweight material. A carrying handle,
designated generally as 62 in FIG. 2, is integrally formed into the
front of housing 60. With handle 62, programmer 20 can be carried
like a briefcase.
An articulating display screen 64 is disposed on the upper surface
of housing 60. Display screen 64 folds down into a closed position
(not shown) when programmer 20 is not in use, thereby reducing the
size of programmer 20 and protecting the display surface of display
64 during transportation and storage thereof.
A floppy disk drive is disposed within housing 60 and is accessible
via a disk insertion slot (not shown). A hard disk drive is also
disposed within housing 60, and it is contemplated that a hard disk
drive activity indicator, (e.g., an LED, not shown) could be
provided to give a visible indication of hard disk activation.
As would be appreciated by those of ordinary skill in the art, it
is often desirable 30 to provide a means for determining the status
of the patient's conduction system. To accomplish this task and
provide suitable ECG tracings, programmer 20 is equipped with
external ECG leads 24.
In accordance with the present invention, programmer 20 is equipped
with an internal printer (not shown) so that a hard copy of a
patient's ECG or of graphics displayed on the programmer's display
screen 64 can be generated. Several types of printers, such as the
AR-100 printer available from General Scanning Co., are known and
commercially available.
In the perspective view of FIG. 2, programmer 20 is shown with
articulating display screen 64 having been lifted up into one of a
plurality of possible open positions such that the display area
thereof is visible to a user situated in front of programmer 20.
Articulating display screen is preferably of the LCD or
electro-luminescent type, characterized by being relatively thin as
compared, for example, a cathode ray tube (CRT) or the like.
As would be appreciated by those of ordinary skill in the art,
display screen 64 is operatively coupled to the computer circuitry
disposed within housing 60 and is adapted to provide a visual
display of graphics and/or data under control of the internal
computer. Programmer 20 described herein with reference to FIG. 2
is described in more detail in U.S. Pat. No. 5,345,362 issued to
Thomas J. Winkler, entitled "Portable Computer Apparatus With
Articulating Display Panel," which patent is hereby incorporated
herein by reference in its entirety. The Medtronic Model 9790
programmer is the implantable device-programming unit with which
the present invention may be advantageously practiced.
FIG. 3 is a block diagram of the electronic circuitry that makes up
pulse generator 10 in accordance with the presently disclosed
invention. As can be seen from FIG. 3, pacemaker 10 comprises a
primary stimulation control circuit 20 for controlling the device's
pacing and sensing functions. The circuitry associated with
stimulation control circuit 20 may be of conventional design, in
accordance, for example, with what is disclosed U.S. Pat. No.
5,052,388 issued to Sivula et al., "Method and apparatus for
implementing activity sensing in a pulse generator." To the extent
that certain components of pulse generator 10 are conventional in
their design and operation, such components will not be described
herein in detail, as it is believed that design and implementation
of such components would be a matter of routine to those of
ordinary skill in the art. For example, stimulation control circuit
20 in FIG. 3 includes sense amplifier circuitry 24, stimulating
pulse output circuitry 26, a crystal clock 28, a random-access
memory and read-only memory (RAM/ROM) unit 30, and a central
processing unit (CPU) 32, all of which are well-known in the
art.
Pacemaker 10 also includes internal communication circuit 34 so
that it is capable communicating with external programmer/control
unit 20, as described in FIG. 2 in greater detail.
With continued reference to FIG. 3, pulse generator 10 is coupled
to one or more leads 14 which, when implanted, extend transvenously
between the implant site of pulse generator 10 and the patient's
heart 16, as previously noted with reference to FIG. 1. Physically,
the connections between leads 14 and the various internal
components of pulse generator 10 are facilitated by means of a
conventional connector block assembly 11, shown in FIG. 1.
Electrically, the coupling of the conductors of leads and internal
electrical components of pulse generator 10 may be facilitated by
means of a lead interface circuit 19 which functions, in a
multiplexer-like manner, to selectively and dynamically establish
necessary connections between various conductors in leads 14,
including, for example, atrial tip and ring electrode conductors
ATIP and ARING and ventricular tip and ring electrode conductors
VTIP and VRING, and individual electrical components of pulse
generator 10, as would be familiar to those of ordinary skill in
the art. For the sake of clarity, the specific connections between
leads 14 and the various components of pulse generator 10 are not
shown in FIG. 3, although it will be clear to those of ordinary
skill in the art that, for example, leads 14 will necessarily be
coupled, either directly or indirectly, to sense amplifier
circuitry 24 and stimulating pulse output circuit 26, in accordance
with common practice, such that cardiac electrical signals may be
conveyed to sensing circuitry 24, and such that stimulating pulses
may be delivered to cardiac tissue, via leads 14. Also not shown in
FIG. 3 is the protection circuitry commonly included in implanted
devices to protect, for example, the sensing circuitry of the
device from high voltage stimulating pulses.
As previously noted, stimulation control circuit 20 includes
central processing unit 32 which may be an off-the-shelf
programmable microprocessor or micro controller, but in the present
invention is a custom integrated circuit. Although specific
connections between CPU 32 and other components of stimulation
control circuit 20 are not shown in FIG. 3, it will be apparent to
those of ordinary skill in the art that CPU 32 functions to control
the timed operation of stimulating pulse output circuit 26 and
sense amplifier circuit 24 under control of programming stored in
RAM/ROM unit 30. It is believed that those of ordinary skill in the
art will be familiar with such an operative arrangement.
With continued reference to FIG. 3, crystal oscillator circuit 28,
in the presently preferred embodiment a 32,768-Hz crystal
controlled oscillator provides main timing clock signals to
stimulation control circuit 20. Again, the lines over which such
clocking signals are provided to the various timed components of
pulse generator 10 (e.g., microprocessor 32) are omitted from FIG.
3 for the sake of clarity.
It is to be understood that the various components of pulse
generator 10 depicted 10 in FIG. 3 are powered by means of a
battery (not shown) that is contained within the hermetic enclosure
of pacemaker 10, in accordance with common practice in the art. For
the sake of clarity in the Figures, the battery and the connections
between it and the other components of pulse generator 10 are not
shown.
Stimulating pulse output circuit 26, which functions to generate
cardiac stimuli 15 under control of signals issued by CPU 32, may
be, for example, of the type disclosed in U.S. Pat. No. 4,476,868
to Thompson, entitled "Body Stimulator Output Circuit," which
patent is hereby incorporated by reference herein in its entirety.
Again, however, it is believed that those of ordinary skill in the
art could select from among many various types of prior art pacing
output circuits that would be suitable for the purposes of
practicing the present invention.
Sense amplifier circuit 24, which is of conventional design,
functions to receive electrical cardiac signals from leads 14 and
to process such signals to derive event signals reflecting the
occurrence of specific cardiac electrical events, including atrial
contractions (P-waves) and ventricular contractions (R-waves). CPU
provides these event-indicating signals to CPU 32 for use in
controlling the synchronous stimulating operations of pulse
generator 10 in accordance with common practice in the art. In
addition, these event-indicating signals may be communicated, via
uplink transmission, to external programming unit 20 for visual
display to a physician or clinician.
Those of ordinary skill in the art will appreciate that pacemaker
10 may include numerous other components and subsystems, for
example, activity sensors and associated circuitry. The presence or
absence of such additional components in pacemaker 10, however, is
not believed to be pertinent to the present invention, which
relates primarily to the implementation and operation of
communication subsystem 34 in pacemaker 10, and an associated
communication subsystem in external unit 20.
FIG. 4 is a ladder diagram of the ADI/R operation, specifically a
Marker Channel.RTM. Diagram. With the help of the (pre-2002) NBG
Code, one familiar with the state of the art will be able to
discern that the letter in the first position (A) means that the
pacemaker (or other implanted device) will pace the atrium in the
absence of an atrial sensed event. The second letter (D) implies
that the pacemaker will sense in dual chambers, that is, both the
atrial and ventricular chambers. The third letter (I) means that,
upon sensing in either chamber, pacing will be inhibited in that
specific chamber. The final letter, R, implies that the device may
be rate responsive that is, altering the atrial rate in response to
an artificial sensor, such as a Piezo-electrical crystal,
accelerometer, minute ventilation, etc.
The operation of the preferred ADI/R mode is depicted in the ladder
diagram as follows. Atrial paced (or sensed) event 1 initiates a
non-programmable, auto 15 adjusting (e.g., 100-150 millisecond)
blanking period 4, followed by auto-adjusting atrial sensitivity
(not shown). Sensing circuitry (see FIG. 3) determines if and when
ventricular sensed event 2 has occurred. If detected, timing
circuitry (see FIG. 3) initiates VA interval 9. Other timing,
blanking periods, and refractory periods serve the following
purposes. A programmable ventricular blanking period 8 prevents
sensing of atrial pace 1 on the ventricular channel, sometimes
termed "cross-talk." Ventricular sensed event 2 starts a 120
millisecond post ventricular atrial blanking (PVAB) period 6,
followed by auto-adjusting atrial sensitivity. PVAB 6 serves the
purpose of preventing sensing of the R-wave or T-wave on the atrial
channel, termed "far-field R-wave sensing." Ventricular sensed
event 2 also starts 100 millisecond ventricular blanking 7 followed
by auto-adjusting ventricular sensitivity. This period serves the
purpose of preventing sensing of the ventricular output pulse or
the ventricular depolarization itself. Repolarization, or T-wave 3,
follows R-wave 2. Ventricular event 2 detected by sensing circuitry
(see FIG. 3) sends signal to timing circuitry to start VA interval
9, leading to the next atrial pacing cycle. Two R-R intervals are
depicted in FIG. 4. As described in more detail hereinbelow, an ARP
may have a nominal value of approximately seventy percent (70%) of
a single preceding R-R interval (in a beat-to-beat implementation)
or of a series of preceding R-R intervals.
Taking into account that this mode would be used primarily with
Sick Sinus patients who have full or some degree of intact AV
conduction, this type of operation as depicted for the ADI/R mode
is something the clinician or physician would expect to occur. In
the presence of relatively reliable intact AV conduction the
pacemaker will maintain the ADI/R operation/mode. Sensed
ventricular events would occur in the vast majority of cardiac
cycles (that is, PQRST). FIG. 5 teaches what will occur should the
patient develop transient AV block for one or a few cardiac
cycles.
FIG. 5 is a ladder diagram of the DDI/R operation in the event that
the patient experiences a PVC, in one embodiment. The purpose of
the DDI/R operation is to maintain ventricular support (i.e., help
the patient recover sufficient cardiac output following the PVC).
Briefly stated, the implanted device mode switches from the
preferred ADI/R to the DDI/R for in response to a detected PVC for
at least one cardiac cycle.
The timing of the DDI/R is as follows. In the DDI/R mode (third
pacing cycle, labeled DDI/R), AV interval 5 is set to a short
period (e.g., 80 milliseconds), following the paced P-wave due to
the presence of a PVC between the second and third atrial paced
events. The purpose of this short AV interval 5 is intended to
suppress competition between ventricular pacing pulse culminating
in paced R-wave 13 and any potential intrinsic R-wave with a
delayed conduction from the previous paced atrial event. Assuming
the presence of such an intrinsic R-wave, the timing of the
ventricular output pulse would normally result in a ventricular
pacing pulse falling into the absolute refractory period of the
intrinsic, conducted R-wave, resulting in a psuedo-fusion beat (not
shown). This operation is intended to prevent the onset of a
ventricular tachycardia, should the ventricular pacing pulse fall
into the relative refractory period of the ventricle, commonly
called "pacing on T" phenomenon. In this respect, the reader is
again cautioned that the drawings do not necessarily reflect actual
or practical timing, but are intended to illustrate the notion of a
mode switch (to DDI/R) following a PVC.
With respect to the foregoing, in one form of the invention, if the
Ap encroaches on the preceding Vs (e.g. within 300 msec) for more
than about four 30 depolarization events (e.g., consecutive beats),
then the pacing rate is decreased. In effect, this creates a
dynamic upper sensor rate. Thus, the present invention addresses an
anticipated concerns with regard to the MVP modality providing
relatively short VS-AP intervals. Such intervals could cause
disadvantageous patient symptoms and may also have a negative heart
remodeling effect. To counter these issues the MVP modality can
operate such that after a V-Sense event (Vs), a scheduled A-Pace
(Ap) event is delayed until some pre-defined interval expires. This
aspect of the MVP modality is somewhat similar to upper tracking
rate (UTR) hold off or non-competitive atrial pacing (NCAP)
hold-off except that it is based on an A-Pace (Ap) event following
a V-Sense (Vs). This results in the atrium being paced at a
slightly lower rate than intended which may create issues that are
known to exist with respect to so-called atrial overdrive pacing
algorithms. This aspect of the MYP modality is preferably
implemented in hardware (just like UTR and NCAP) primarily because
of the critical timing involved.
In order to prevent adverse hemodynamics that may result from
atrial pacing soon (e.g. within 250 msec) after a ventricular sense
(i.e. Vp-As) while in the preferred ADI/R mode of pacing, one
option is to (and subsequently limit for a period of time (e.g. one
hour) the sensor driven pacing rate in the event of continuous
cycles (e.g. 4-8 consecutive) of atrial pacing within a
programmable interval (e.g. 250 msec) of the preceding R-waves. For
example, such a dynamic upper rate limit is preferably set so that
the Vs-Ap interval does not decrease to less than about 300 ms.
Continuing with the timing in FIG. 5, paced R-wave 13 starts a 120
millisecond ventricular blanking period 7, followed by auto
adjusting ventricular sensitivity (not shown). Paced R-wave 13 also
starts a 120 millisecond PVAB 6 followed by auto adjusting atrial
sensitivity (not shown). Assuming the transient AV block
self-corrects and a sensed R-wave is detected in response to the
ventricular pace (Vp), the preferred ADI/R resumes with the next
paced or sensed P-wave, as is depicted in FIG. 4.
FIG. 6 is a ladder diagram that depicts the pacing operation in the
event that the patient develops AV block for more than one cycle.
Note that according to the preferred embodiment of the present
invention, a single missed beat (i.e., no Vs) will not by itself
cause a mode switch, particularly if relatively reliable AV
conduction is present. Following a mode switch to DDI/R, VA
interval 9 times out, resulting in atrial paced event 1. A very
long (e.g. 400 millisecond or up to approximately 70% of the median
V-V interval) 17 may be used in an attempt to promote native AV
conduction (or a Vp stimulus may be withheld) as further described
hereinbelow. If, however, AV interval 17 is not interrupted by a
sensed, intrinsic R-wave, as is depicted in the first cycle
(labeled ADI/R), the pacemaker immediately switches to the DDD/R
mode. In the event that a sensed, intrinsic R-wave does occur, the
device reverts to the ADI/R operation (not shown). The DDD/R
operation, with the programmed AV interval, will be sustained until
a sensed, intrinsic R-wave is detected, as further described
herein. Periodic attempts to force restoration of the ADI/R
operation may be performed (as depicted in FIG. 7). Mode switching
to the DDI/R mode may occur in the event that a PVC is detected and
in the event that that an atrial tachycardia is detected a mode
switch to DDD/R pacing is preferred.
FIG. 7 is a ladder diagram that depicts a periodic attempt to
restore the ADI/R operation during sustained DDD/R operation. As
mentioned, the DDD/R mode may become the sustained mode of
operation in the event that the patient develops a prolonged AV
block, such as might occur with rate-dependent AV block or if the
AV conduction become relatively unreliable. In such cases, the
device may be programmed to revert to ADI/R 1 after a programmable
number of DDD/R cycles. Then, the device looks for a ventricular
sensed event, e.g., at 23 following atrial pace 1. In the event
that a sensed, intrinsic R-wave is detected, the ADI/R operation is
immediately resumed. In the absence of a ventricular sensed event,
the device continues to operate in the DDD/R mode, as indicated in
the third cycle of FIG. 7.
FIG. 8 is a ladder diagram of the pacing operation in the event
that the patient 20 develops an atrial tachycardia. A sick sinus
patient often has episodes of atrial tachycardia, atrial flutter,
or atrial fibrillation. During these episodes, the pacing operation
must be set such that the ventricular pacing rate will neither be
synchronized to the fast atrial rate nor so slow as to cause
symptoms. Preferably during episodes of AT, the atrial-based pacing
ends and a DDD/R (or DDI/R) pacing mode is employed.
In FIG. 5 it was noted that the device, while operating in/mode
also is well suited for pacing in the presence of an atrial
tachycardia because it will not allow ventricular synchronization
to a fast atrial rate nor will it allow the ventricular pacing rate
to go below the programmed lower rate. Therefore, when an atrial
tachycardia does occur, as shown in FIG. 8, fast atrial sensed
events 27 without a conducted ventricular event have no effect on
ventricular timing 9. Since there is no ventricular event, the
operation immediately switches to the DDI/R mode. In the presence
of an atrial tachycardia, the V-V interval 9 times out so that
paced R-wave 8 will occur at the faster of the programmed lower
rate or sensor-indicated rate in the DDI/R mode. The operation
depicted in FIG. 8 will continue so long as the atrial tachycardia
persists. Upon termination of the atrial tachycardia, the preferred
ADI/R will resume as shown in FIG. 4 or 7, depending on how the
heart recovers from the atrial tachyarrhythmia. If the atrial
tachyarrhythmia terminates abruptly, the prompt restoration of the
ADI/R mode may take place (see FIG. 4). If, however, the atrial
tachyarrhythmia "cools down" slowly, there may be a period of DDD/R
pacing with periodic attempts to restore ADI/R pacing as shown in
FIG. 7.
In contrast to a majority of the foregoing, and with general
reference to FIG. 9, the MVP modality includes one or more of the
following aspects.
Adaptive Atrial Refractory Period (ARP)
According to the initial definition of the preferred ADI/R
modality, a rate-adaptive ARP is employed in order to distinguish
physiologic atrial events from non physiologic events. According to
a preferred implementation, an adaptive ARP is employed and defined
as a fixed percentage of the physiologic interval (P1). One
preferred method of determining the P1 is based on the ventricular
rate as determined by the median R-R interval for the preceding 12
ventricular events (regardless if such events are sense- or
pace-type events). Specifically the median value is determined
algorithmically as the seventh longest interval of the preceding 12
R-R (i.e., V-R, RV, or V-V) intervals. Therefore, recalculation of
the P1 occurs following event ventricular event as a new interval
is added to a 12 beat accumulator (e.g., temporary memory
structure) and the oldest is eliminated according on a FIFO
(first-in, first-out) basis. Of course, a beat-to-beat
instantiation may be used in lieu of the multi-beat techniques
described herein.
The preferred implementation defines ARP as a programmable, fixed
percentage of the P1. A suggested default value is seventy percent
(70%) of the R-R interval (either a calculated value--such as a
median value--or a beat-to-beat value derived from a prior R-R
interval). Thereby, intrinsic atrial events that occur at regular
intervals (consistent with a patient's current physiologic state)
that fall outside of the ARP can be defined as physiologic while
those within the ARP can be assumed to represent noise or are
otherwise not physiologic. Alternatively, the ARP can be
implemented as an adaptive approach with a fixed, absolute time
period (i.e., fixed period of time maintained for the remainder of
the P1). The philosophy behind the latter approach is for avoidance
of atrial competitive pacing during the physiologic refractory
period of the atrium. One possible downside of a fixed (e.g., 300
ms) alert period outside of the ARP, however, is the increased risk
of misclassifying non 5 physiologic atrial events as
physiologic.
Mode Supervisor:
The Wenckebach supervisor (as briefly described previously) has
been renamed the "mode supervisor" because the mode supervisor can
control a wide range of operations related to mode changes. The
primary intent of the mode supervisor is to monitor a patient's
atrioventricular status and intervene when necessary by invoking
sustained mode-switches to conventional modes of pacing (i.e. DDD/R
and DDI/R). According to the preferred implementation, the mode
supervisor defines unreliable AV conduction according to a
Wenckebach pattern with definition of a critical AV conduction
acceptance ratio to discriminate between tolerable (or "relatively
reliable") AV conduction states from intolerable (or "relatively
unreliable") AV conduction states. For instance, an AV conduction
acceptance ratio of 4:3 allows preferred ADI/R operation to persist
as long as there are at least three ventricular events for every
four physiologic atrial events. Should AV conduction falter such
that the ratio of A to V events falls below the pre-defined
acceptance ratio, a sustained switch to conventional DDD/R pacing
will occur. Importantly, atrial events classified as
non-physiologic (i.e. within the ARP) are not accounted for in the
calculation of the A:V ratio. Thereby, inappropriate mode-switches
to DDD/R are avoided in the presence of frequent non-conducted
premature atrial contractions (PAC).
Upon invoking DDD/R pacing in the presence of unreliable AV
conduction, the mode supervisor immediately assumes the role of
striving to restore preferred ADI/R pacing. Since it is known that
AV conduction disease typically progresses gradually with brief
manifestations of high degree block expected in the early stages of
disease progression, the mode supervisor will attempt to restore
preferred ADI/R operation following only a brief episode of new
onset DDD/R pacing. According to the preferred operation, the first
reattempt to reveal intact AV conduction and to restore ADI/R
pacing will occur only after a short period of time (e.g., one
minute) of DDD/R pacing. Should ADL'R restoration fail, reattempts
will be attempted at 2, 4, 8, 16 and 32 minutes and subsequently at
1, 2, 4, 8, 12 and 24 hours. Of course, other timing sequences may
be used, both periodic and aperiodic (as well as local and remote
clinician- or patient-activated atrial-based pacing
initiation).
The algorithm used to search for intact AV conduction and restore
ADI/R is defined according to one of two options. The first option
is to simply withhold a ventricular pace stimulation during DDD/R
operation. In the event that a ventricular sense follows the
physiologic atrial event during which ventricular pacing was
withheld, ADI/R pacing is resumed. Otherwise, DDD/R pacing
continues with subsequent reattempts according to a schedule or by
way of manual activation (as specified above). The second option
searches for intact AV conduction involves extending the AV delay
during DDD/R pacing to a pre-designated AV conduction [search]
interval (AVCI). For instance, with an AVCI of 400 ms, the AV delay
is extended to 400 ms following a physiologic atrial event (sensed
or paced). In the event that the AV interval is interrupted by a
ventricular sense, thereby preempting the ventricular pace in DDD/R
operation, the mode supervisor reverts to ADI/R operation.
Otherwise, a ventricular pace is delivered upon the expiration of
the AVCI interval and DDD/R operation resumes with reattempts
according to the schedule (or with manual activation) as described
above. Importantly, in the event of failed conduction and
ventricular pacing during these AV conduction search methods, an
extended post-ventricular atrial refractory period (PVARP) in
invoked following the AVCI in order to guard against the
possibility of retrograde conduction initiating a pacemaker
mediated tachycardia.
A third responsibility of the mode-supervisor is to recognize
sustained pathologic atrial rhythms and to invoke sustained
mode-switching to DDI/R pacing for the duration of the atrial
tachyarrhythmia (AT). It is expected that the defining AT criteria
will be consistent with that used by conventional pacing modes
(e.g. 4 of 7 short A-A intervals) and that mode-switching operation
will not be unique to the minimum ventricular pacing (MYP) modality
and therefore is not further described in this disclosure. The
uniqueness of the implementation within MVP lies in the possibility
that mode-switching to DDI/R will occur either from the ADI/R or
DDD/R operating states. The inventors believe that the notion of
switching to/from DDI/R is novel, and although not practically
necessary as ADI/R is not an atrial tracking mode, there may be
some merit to switching directly to DDI/R in order to avoid an
inadvertent switch to DDD/R in the event of transient conduction
block during an AF event. Moreover, a sustained switch to DDI/R may
be justified in order to provide some degree of rate-regularization
during AF with an irregular ventricular response.
A fourth responsibility of the mode supervisor is to monitor for
rapid repeated switches between preferred ADI/R and DDD/R pacing
modes. If the device repeatedly switches back and forth between
these modes every minute or every two minutes (e.g., or other
relatively short period of time) the mode supervisor can suspend
testing for AV conduction and allow the device to remain in DDD/R
pacing, for example by setting the AV conduction testing interval
to some number of hours (e.g. 2, 4, 8, 16). The number of repeated
mode changes required to trigger such behavior remains to be
determined and may be programmable.
A fifth responsibility of the mode supervisor is to monitor for
repeated failed AV conduction tests at maximal test duration. So
for example, if seven straight tests for AV conduction fail at 16
hour intervals, the mode supervisor can suspend AV conduction
testing and the device can then remain in the DDD/R mode
indefinitely.
A sixth responsibility of the mode supervisor involves suspending
AV conduction testing based on physiologic parameters (rather than
indefinitely terminating searches or simply suspending for a fixed
number of hours or other period of time). For example, the mode
supervisor can monitor heart rate and recognize that repeated
switching back and forth between preferred ADI/R and DDD/R is
associated with high heart rates (HR) or activity, and suspend AV
conduction testing until the HR returns below a preset or
dynamically set HR threshold. Similar functionality can be
implemented in the case of rapid repeated switching associated with
just low heart rates.
A seventh responsibility of the mode supervisor relates to varying
the tolerated Wenckebach threshold dependent on the time of day or
a signal from a sleep indicator. For example, in patients with
known incidence of Wenckebach during sleep, the supervisor changes
the threshold to tolerate more severe Wenckebach at that time in
response to a positive indication that the patient has entered a
sleep state or simply as a matter of timing (e.g., increase
Wenckebach tolerance during expected sleep time of the
patient).
An eighth responsibility of the mode supervisor involves
maintaining a record of the sensor driven atrial paced rate at
which the Wenckebach threshold was exceeded during ADI/R operation
(thereby causing a mode switch to DDD/R). Subsequently, the upper
sensor rate is thus restricted to not encourage high rate sensor
driven pacing above rates at which reliable AV conduction does not
exist. This operation, in essence, is a dynamic upper sensor rate
that adapts according to information obtained during mode
excursions from ADI/R to DDD/R.
A ninth responsibility of the mode supervisor relates to
controlling the pacing mode of an ICD following delivery of a
defibrillation therapy to the patient (i.e., high voltage shock
delivery). In this aspect of the invention, the mode supervisor
initiates ADI/R pacing with a DDI sequence, or in the ADI/R mode at
a premature timing interval following delivery of a defibrillation
therapy (i.e., a high voltage shock) in order to prevent a
significant delay in delivery of a ventricular pace (Vp) in the
event of transient post-shock AV block. Alternatively, a preferred
option favors DDD/R pacing and delays resumption of ADI/R pacing
for a pre-specified period of time following delivery of such a
high voltage defibrillation shock.
PVC Response
According to ADI/R operation, premature ventricular contractions
(PVC) will 20 not alter the timed delivery of the ensuing atrial
pace. Since this can conceivably result in a closely coupled
conducted ventricular event due to atrial pacing coincident or soon
following a PCV, the inventors decided to deviate from ADI/R
operation in this circumstance, in some embodiments, and
effectively operate in a DDI/R modality. In doing so, following a
PVC event the ensuing atrial pace is delayed and scheduled
according the operating AV delay (preferably equal to the P1 minus
80 ms). In addition to providing more appropriate rhythm responses
during bradycardia pacing operation interrupted by PVCs, the added
advantage of having this PVC response is that asynchronous atrial
pacing is avoided during runs of ventricular tachycardia. This has
especially important implications for tachyarrhythmia control
devices, which typically require consecutive detected VT intervals,
as withholding atrial pacing during VT also removes the potentially
interfering cross-chamber ventricular blanking periods that occur
with atrial pacing.
Various aspects of certain embodiments of the present invention can
be implemented using executable software code and/or operational
parameters saved by (or downloaded to) a medical device. Such a
device may be disposed in vivo and later programmed according to
the invention or may be programmed prior to implantation (e.g.,
using firmware that may be reprogrammed or modified using telemetry
techniques and the like). This is in contrast to a beat-to-beat
implementation of the invention, which would preferably be
implemented in hardware as understood by those of skill in the art.
However, the present invention is not limited to only firmware or
hardware implementations; indeed, the present invention may be
implemented in a hybrid or combined in any desirable manner using
device programming techniques known and used in the art. For
clarity, however, the inventors specifically provide and herein
claim a beat-to-beat instantiation of the present invention wherein
the operation of the MVP modality is invoked for every beat on a
beat-to-beat basis.
Referring to FIGS. 10A and 10B, the above referenced MVP protocols
will generally handle cross talk and PVC's in the following manner.
In FIG. 10A, an atrial pace is delivered during interval 1 and an
atrial blanking period ABP or similar window is defined. Typically,
such a window is on the order of about 80 ms but may be defined as
desired. Any ventricular event, whether intrinsic or far-field is
ignored if it occurs during the ABP. Thus, during interval 2 a PVC
occurs within the ABP and is ignored. Despite there having been an
intrinsic, conducted ventricular event the protocol considers
interval 2 to be devoid of ventricular activity. Depending upon the
embodiment of MVP, ventricular pacing is provided in interval 3 and
in subsequent intervals until a conduction check occurs, a
ventricular pace is inhibited, or another event occurs to switch
the device back to the atrial based mode. In other words, despite
there being intrinsic conduction present, ventricular pacing is
provided; thus, reducing the efficiency of MVP in eliminating as
much ventricular pacing as possible.
FIG. 10B illustrates cross talk or far field sensing of the atrial
pace that is sensed by the ventricular lead and indicated on the
ventricular channel during the ABP. As these events are ignored,
this will have no effect on the MVP. Furthermore, the ABP is
successful in preventing such cross talk from being misinterpreted.
In other words, in this instance the ABP is performing its intended
function and does not negatively affect the protocol for minimizing
or reducing ventricular pacing. Thus, while cross-talk is
appropriately handled, PVC's or other intrinsic ventricular events
that occur during the ABP reduce the efficiency.
To avoid this reduction in efficiency, the present invention
provides "feed back" or "feed forward" cross talk filtering
protocols that are implemented with MVP. It is understood that
there are many embodiments of MVP and that each will tolerate or
react to missed ventricular beats in a variety of ways. The
following description is meant to apply to any of these
embodiments, though each variation is not separately described in
detail.
FIG. 11A illustrates an embodiment using a "feed back" cross talk
filtering protocol to address this situation. Under the "feed back"
protocol, an event sensed in the ABP during an interval is
classified as invalid, and hence ignored. This classification
information is stored in hardware, software, firmware, or memory as
a marker or other indicator. The ABP of the subsequent cycle (e.g.,
3) is monitored. If there is a similar sense during this ABP, then
the earlier classification is maintained. If there is no similar
sense during the ABP then the earlier event is "reclassified" as a
valid ventricular event and treated as such by the MVP algorithm.
For example, in interval 2, a PVC occurs during the ABP. Initially,
this is classified as invalid. During the next interval 3, there is
no sensed event during the ABP and at the expiration of the ABP the
previous PVC is reclassified as valid. Thus, interval 2 is
indicated to have intrinsic ventricular conduction as of the
expiration of the ABP in interval 3. As such, no ventricular pacing
is provided during interval 3. From this point forward, operation
continues normally under MVP.
In practice, this reclassification can be implemented in any number
of ways. For example, if MVP is utilizing actual mode switching,
the appropriate mode switch (assuming no ventricular sense in
interval 2) could occur at the atrial pace for interval 3 with a
subsequent mode switch (either actual or effective) at the
expiration of the ABP. Alternatively, the mode switch to the dual
chamber mode could be set to occur only after the ABP if there is
no reclassification. Additionally, utilizing flags would include
setting an appropriate flag after the ABP with the subsequent
functionality resulting. As all of this occurs in the interval of
interest, various ad hoc or single interval steps may be taken to
address the implications of the short timeline leading to a return
to normal MVP operation.
FIG. 11B illustrates a case where cross talk occurs during the ABP
of interval 2. Again, since this is the first occurrence this
ventricular sense is classified as invalid. During the subsequent
ABP which occurs in interval 3, far field sensing will most likely
occur again and is illustrated as such. Therefore, the ventricular
sense of interval 2 is not reclassified, but rather remains invalid
and ignored. MVP will function based upon whether or not
ventricular events are sensed outside of the ABP.
Under extremely rare circumstances, this may lead to short term
anomalous behavior that is either tolerated or in some embodiments
addressed. It is conceivable (FIG. 11B) that far field sensing
occurs only during interval 2 and the sensed event in interval 3 is
actually a PVC or other intrinsic event. One option is to simply
ignore this extremely unlikely scenario and simply treat the PVC as
a far field sense. Another option would be to scrutinize the timing
within the ABP of the various ventricular sensed events. Far field
sensing should have predictable timing; thus, a PVC may fall within
the ABP but have sufficiently different timing to distinguish this
event so as to either reclassify the event of interval 2
(erroneously) or cause the event of interval 3 to be evaluated
differently in intervals 4 and beyond. In other words, timing may
be used to further distinguish ventricular senses during the ABP
from binary results to more analytical results to distinguish
between events when cross talk and PVC's are occurring.
Furthermore, if far field sensing occurs along with and is
separately distinguishable from a PVC during the ABP, this could
also be used to indicate that intrinsic conduction is present along
with cross talk.
FIGS. 12A and 12B illustrate a "feed forward" cross talk filtering
protocol for addressing sensed events during the ABP. In FIG. 12A,
a PVC occurs during the ABP of interval 2. In this protocol, the
first occurrence of such an event is classified as valid; in other
words, it is considered an intrinsic, conducted event. Moving
forward in time, MVP will behave normally from that point on. Thus,
any sensed event on the ventricular channel is treated as a valid
by MVP, regardless of when it occurs during the interval. While
providing the desired results in the efficiency of MVP, this alone
would simply be equivalent to eliminating the ABP and would leave
open the problems associated with far field sensing or cross
talk.
FIG. 12B illustrates how this problem is addressed. Here, the
ventricular sense during the ABP of interval 2 is cross talk. As
indicated, this occurrence is classified or considered as a valid
ventricular event. Thus, whether or not an intrinsic ventricular
depolarization occurs during interval 2, the MVP protocol acts as
if it has. The classification of this event during the ABP as valid
is "fed forward" and affects subsequent senses during the ABP. In
this example, cross talk is sensed during the ABP of interval 3.
The "fed forward" indicator has been toggled and this sense during
the ABP of interval 3 is classified as invalid. Assuming cross talk
is sensed during subsequent ABPs, those events will also be
considered invalid. Thus, only a ventricular sense occurring
outside of the ABP will satisfy the MVP protocol in interval 3 and
beyond.
This may be a one-time toggle with an assumption that once cross
talk is sensed, it will remain an issue until manually changed or
corrected. Alternatively, the protocol may include provisions to
reset itself if a predetermined number of intervals transpire
without sensed activity during the ABP. This will automatically
address intermittent cross talk. The predetermined number of
intervals may be fixed or vary. That is, for the first occurrence,
the system may reset after a few intervals but if cross talk is
observed with some frequency, then attempts to reset are made less
frequent and/or eliminated.
Similarly to the "feed back" protocol, the "feed forward" protocol
conceptually permits some extremely unlikely anomalous behavior.
First, PVCs may occur in consecutive cycles and be misinterpreted
as cross talk. One solution is the automatic and/or periodic reset
indicated above. Another option is to monitor the timing of the
event within the ABP. True far field sensing should be somewhat
stable and predictable. Thus, if the sensed events vary in time
within the ABP by more that a predetermined percentage, these
events may be reclassified as PVCs or at least subjected to a
higher level of evaluation. Similarly, both far field sensing and
PVCs could be present. This is simply solved by applying the above
classifications, but if multiple events occur during a single ABP
consider one to be an intrinsic ventricular event, subject to the
remaining protocol parameters.
An even more remote scenario could occur in that a patient has
intrinsic conduction and no cross talk for some period of time,
e.g., up through interval 1 of FIG. 12B. Then, during the ABP of
interval 2, a far field sense occurs. As indicated, with the "feed
forward" protocol, this far field sense is classified as valid. In
the same interval, the patient's conduction fails and there is no
intrinsic ventricular activity. However, since the far field sense
is classified as valid, the MVP protocol considers interval 2 to
have a ventricular event. Subsequently, in interval 3, an atrial
pace AP is delivered. During this ABP there could be another far
field sense, which is classified as invalid due to the "feed
forward" indicator. Again, assuming conduction block has occurred
there is no intrinsic conduction; thus, there is no ventricular
depolarization. At the conclusion of interval 3, the MVP protocol
will recognize the absence of conduction and respond accordingly.
Typically, this means providing a ventricular pace. However, in
some embodiments, the mode supervisor MS may seek to have more than
one interval out of a predetermined number of intervals without
ventricular activity. Since, interval 3 was the first interval
considered to be devoid of ventricular activity, the mode
supervisor MS could permit interval 4 to transpire without
ventricular pacing. Thus, it would be conceivable to have two or
even three intervals without ventricular activity.
There are several ways to address this issue. The first is to
realize that the likelihood of initiating far field sensing and
loosing intrinsic conduction during the same cycle are miniscule.
Even if this were to result, the effect on the patient would be
tolerable and pacing would result in the third or fourth interval.
Thus, while conceptually possible, the practical likelihood and
results render this issue essentially moot.
To prevent even the chance of this occurring, certain steps may be
taken. For example, only certain embodiments of the mode supervisor
MS will permit a situation where three intervals could conceivably
transpire without ventricular activity. Thus, one option is to not
use this aspect of the mode supervisor protocol with the "feed
forward" protocol.
Another option is to utilize portions of the "feed back" protocol
with the "feed forward" protocol in certain circumstances. For
example, the far field sense during the ABP of interval 2 is deemed
valid. Because of this, the far field sense during the ABP of
interval 3 is deemed invalid. When this condition occurs, the mode
supervisor MS may then revisit the previous interval to determine
if a second ventricular event occurred. If not, the validity of the
interval 2 ABP sense may be reclassified.
It is to be understood that the above description is intended to be
illustrative and, not restrictive. Many other embodiments will be
apparent to those of skill in the art upon reading and
understanding the above description. The scope of the invention
should, therefore, be determined with reference to the appended
claims, along with the full scope of equivalents to which such
claims are entitled.
* * * * *